The conservation specialist, which has a network of 11 studios across the UK, revealed a pre-tax profit of £2.7 million for the 12 months to 30 April 2018 in its latest results announcement – up from £2 million in the previous year. Turnover dropped 4 per cent to £19.3 million over the same period.

Purcell said it would become a limited company this May in the first step towards giving employees ownership of the business next year.

Chief executive Mark Goldspink said the change was designed ‘to ensure the ongoing future strength of the business and our ability to continue delivering an exceptionally high-quality service to our clients and collaborators’.

He added: ‘Purcell continues to grow steadily, based on our geographical diversity, both within the UK and internationally, and the depth and strength of our sought-after expertise, including architecture, masterplanning and heritage consultancy.

‘Our work to protect and modernise iconic buildings like Manchester Town Hall, the Elizabeth Tower, the National Portrait Gallery and Tai Kwan in Hong Kong are among numerous projects supporting our revenue and further enhancing our reputation.’

Average monthly headcount at the practice – which dropped three places to 11th in last year’s AJ100 – fell from 283 to 247 in 2017/18. The average number of technical staff shrunk from 216 to 195.

Staff costs fell by 12 per cent to £10.9 million.

The proportion of the practice’s income earned outside the UK grew from 6.2 per cent to 8.0 per cent.

Purcell was recently appointed to the refurbishment of the National Gallery in London, where it will oversee an overhaul of key sections of the Trafalgar Square visitor attraction.

Works will include updating the institution’s largest space – Room 32 – which usually displays large-scale Italian baroque paintings from the 17th century.

Purcell is collaborating with Jamie Fobert Architects on a £35.5 million makeover of the National Portrait Gallery

Purcell is collaborating with Jamie Fobert Architects on a £35.5 million makeover of the National Portrait Gallery

The practice is also working with Jamie Fobert Architects on a £35.5 million makeover of the nearby National Portrait Gallery. Images of this scheme were released last month.

Outside of London, it emerged last year that Purcell had worked up designs for a major refurbishment of Preston’s Harris museum, art gallery and library (pictured).

The employee-ownership structure is becoming increasingly popular with practices in the UK. Assael last month revealed it would be the 18th AJ100 practice to become employee-owned after co-founders John Assael and Russell Pedley turned down a big money offer from the US.

The so-called ‘John Lewis model’ was pioneered in the architecture sector by Make when it launched in 2004.